This invention relates in general to nondestruction evaluation of sample surfaces, and in particular, to an interferometric system for nondestructive evaluation of samples. There is considerable interest in developing analytical tools for analyzing properties of materials, especially in the semiconductor industry. In particular, nondestructive evaluation of semiconductor and other materials will yield valuable information for the design and manufacture of electronic components.
For example, analytical tools suitable for measuring the dose of ion implants can yield valuable information in the design and manufacture of integrated circuit components.
In U.S. Pat. Nos. 4,521,118 and 5,522,510, Rosencwaig proposes a system for measuring thermal waves and thickness of thin films. A laser beam is directed onto a sample periodically to heat the sample. The heating of the sample causes local surface displacement, where such surface displacement causes a change in the local surface slope. The change in the local surface slope is detected by means of a laser probe beam to nondestructively evaluate the properties of the sample.
U.S. Pat. No. 5,298,970 proposes an interferometer for measuring thermal expansion displacement. A sample is heated cyclically by a laser beam which is modulated in intensity. A measuring beam is split into a first beam directed to a mirror as a reference and a second beam directed to the sample. Reflections of the two beams interfere to give a measurement of the thermal displacement.
Another system that can be used for nondestructive evaluation of sample surfaces is spectroscopic ellipsometry, such as the one described in U.S. Pat. No. 5,608,526. While spectroscopic ellipsometry may be useful for measuring doses of arsenic implants, spectroscopic ellipsometry lacks adequate sensitivity for measuring boron implants.
Most of the above-described systems are not entirely satisfactory for nondestructive evaluation of samples, and especially for measuring low dose boron ion implants. It is therefore desirable to provide an improved system with improved characteristics over the above-described techniques.
When a portion of the surface of the sample is heated by means of a beam of radiation, the portion changes its physical characteristics. Such characteristics include reflectivity and local displacement of the surface as well as other physical changes. Such changes in the physical characteristics of the portion of the sample surface is referred to as disturbance of the sample. Thus when a pump beam of radiation is applied to a location of a sample, this results in a disturbance of the sample at that location. Two probe beams of radiation coherent with each other are directed towards the sample, with one probe beam directed to the location of the sample where disturbance is created by the pump beam and the second probe beam to a second location of the sample adjacent to the disturbed location but substantially undisturbed by the pump beam. The two probe beams may have different frequencies or be phase shifted relative to one another. The disturbance may be measured by interfering reflections of the two probe beams at a detector. The disturbance so measured may be compared to a reference for determining the dose of ion implants in a semiconductor material.